In recent years, the organic EL device is paid attention to as a display device for constituting a flat-panel display. The organic EL device generally has a structure in which an organic material is sandwiched between electrodes (an anode and a cathode) from the ups and downs. With a driving current applied to both electrodes, positive holes and electrons are injected respectively from both the electrodes into an organic layer composed of the organic material, and the positive holes and the electrons are recombined with each other in the organic layer, whereby light emission is induced.
As an example of such an organic EL device, there is known a top emission type using the so-called TAC (Top emission Adaptive Current drive) technology, as for example shown in FIG. 1. In the organic EL device shown, a first electrode 2 formed of a light-reflective material and functioning as an anode (positive electrode), an organic layer 3 which is comprised, for example, of a buffer layer 3a, a hole (positive hole) transport layer 3b and an organic light-emitting layer 3c functioning also as an electron transport layer, a second electrode 4 which is comprised of a semi-transparent reflective layer 4a and a transparent electrode layer 4b and functioning as a cathode (negative electrode), and a passivation film 5 formed of a transparent dielectric, are sequentially laminated on a substrate 1, and light from the organic light-emitting layer 3c is taken out to the side of the second electrode 4. Furthermore, the organic EL device has a resonator structure in which the light emitted from the organic light-emitting layer 3c is resonated between the first electrode 2 and the semi-transparent reflective layer 4a of the second electrode 4, and only the light at the resonance wavelength is augmented by the resonator structure so that light with a high peak and a narrow spectrum can be taken out, whereby the color reproduction range of the emitted light can be enlarged.
Besides, as another example of organic EL device, there is known a both-side transparent type one as for example shown in FIG. 2 (Gu et al, Appl. Phys. Lett. 68(19), 1996). In the organic EL device shown, a first electrode 12 having a light transmissive property and functioning as an anode, an organic layer 13 which comprises a hole transport layer 13a and an organic light-emitting layer 13b, and a second electrode 14 which is comprised of a semi-transparent reflective layer 14a and a transparent electrode layer 14b and functions as a cathode, are sequentially laminated on a substrate 11 formed of a transparent glass or the like. Light from the organic light-emitting layer 13b can be taken out to both the sides of the first electrode 12 and of the second electrode 14.
In both of these organic EL devices, the second electrode 4, 14 comprise the semi-transparent reflective layer 4a, 14a as an extremely thin film, in order to enhance a light transmissive property and to secure electrical characteristics. The semi-transparent reflective layers 4a and 14a are formed by co-vapor deposition of magnesium (Mg) and silver (Ag) in an atomic ratio in the range of about 30:1 to 5:1. Namely, the semi-transparent reflective layers 4a and 14a are formed of an alloy containing magnesium as a main constituent.
In the above-mentioned organic EL devices according to the prior art, however, since the semi-transparent reflective layer 4a, 14a is formed of an alloy containing magnesium as a main constituent, the absorption of light in the semi-transparent reflective layer 4a, 14a itself is large, resulting in that the efficiency of taking out the light from the organic light-emitting layer 3c, 13b may be lowered.
For example, when a general magnesium-silver co-vapor deposition film (atomic ratio of about 10:1) is irradiated with light having a wavelength of 550 nm, the reflectance, transmittance and absorbance of light in the co-vapor deposition film are as shown in Table 1 below. Namely, the absorbance is about 16%, even when the thickness of the co-vapor deposition film is varied from 10–15 nm.
TABLE 1FilmReflectanceTransmittanceAbsorbancethickness(%)(%)(%)10 nm51331612 nm43411615 nm602416
When the light absorbance is high, the influence thereof is increased, particularly in the top emission type organic EL device. For example, in the case where the light reflectance on the side of the first electrode 2 is 100, the light from the organic light-emitting layer 3c is transmitted through the second electrode 4, or is reflected by the semi-transparent reflective layer 4a of the second electrode 4 to the side of the first electrode 2, or is absorbed into the semi-transparent reflective layer 4a. It should be noted here that since the light reflectance on the side of the first electrode 2 is 100%, the light reflected to the side of the first electrode 2 is caused by the resonator structure to be again incident on the second electrode 4. Therefore, all the light other than the light portion absorbed into the semi-transparent reflective layer 4a is finally emitted to the outside, irrespectively of the light transmittance and reflectance at the second electrode 4. This means that the efficiency of taking out the light from the organic light-emitting layer 3c is lowered as much as the light absorbance in the semi-transparent reflective layer 4a is large. Even where the reflectance on the side of the first electrode 2 is less than 100%, the stronger the light interference effect due to the resonator structure gets, the more dependant on the absorbance in the semi-transparent reflective layer 4a intend to be.
On the other hand, in the both-side transparent organic EL device, since it has not the resonator structure, only the light portion transmitted through the second electrode 14 is emitted to the outside as the light on the side of the second electrode 14. It should be noted here that the sum total of the light reflectance, transmittance and absorbance is normally constant (100%). Therefore, the light transmittance is influenced as much as the light absorbance is increased. As a result, the efficiency of taking out the light from the organic light-emitting layer 13b may be lowered.
Accordingly, it is an object of the present invention to provide an organic EL device, which enables to obtain sufficient luminance efficiency and the like by suppressing as much as possible the absorption of light so that the light can be taken out efficiently.